Market demand exists for semiconductor integrated circuit elements having higher integration and higher speed. In order to meet this demand, lower-resistance copper wiring has been used in place of conventionally-used aluminum wiring. By combining the copper wiring with a low dielectric constant insulating film (so-called low-k film: insulating film made of material whose dielectric constant is lower than that of silicon oxide) to form multilayer wiring, an integrated circuit element that operates at an extremely high speed can be achieved.
In the manufacturing process of such an integrated circuit element, performing cleaning for the purpose of removing a chemical solution, fine particles, organic substances, and metals attached to the surface of a body to be treated, such as a wafer or a substrate, to achieve and maintain high cleanliness is important for maintaining product quality and enhancing the yield. For example, after treatment with a chemical solution such as a sulfuric acid/hydrogen peroxide mixture or a hydrofluoric acid solution, cleaning with ultrapure water is carried out. In recent years, because of the miniaturization of semiconductor devices, the diversity of materials, and complex processes, the number of cleaning times has been increased. In the formation of the above-mentioned multilayer wiring, the following steps are repeated: forming metal wiring that is a first wiring layer on the substrate; covering the metal wiring with an insulating material; polishing the surface of the insulating material that covers the metal wiring by CMP until it becomes flat; forming metal wiring that is a second wiring layer on the surface; covering the metal wiring with an insulating material; and polishing the surface of the insulating material by CMP until it becomes flat. In such a process, substrate cleaning is carried out after each polishing step.
For the production of the ultrapure water used for the cleaning, generally, an ultrapure water production device shown in FIG. 1 including a pretreatment system, a primary pure water system, and a secondary pure water system (hereinafter, referred to as subsystem) is used. The role of each system in the ultrapure water production device is as follows. The pretreatment system corresponds to a step for removing suspended matters and colloid substances contained in raw water through, for example, coagulation settling and sand filtration. The primary pure water system corresponds to a step for obtaining primary pure water by removing, through the use of, for example, an ion-exchange resin and a reverse osmosis (RO) membrane, ionic components and organic components contained in the raw water from which the suspended matters and the like has been removed by the pretreatment system. The subsystem corresponds to, as shown in FIG. 2, a step for producing ultrapure water by further increasing the purity of the primary pure water obtained by the primary pure water system through the use of a water passage line in which an ultraviolet oxidation device (UV), a non-regenerative ion-exchange device (e.g., cartridge polisher (CP)), a membrane separation device (e.g., ultrafiltration device (UF)), and the like are continuous. The ultrapure water supplied from the subsystem is fed to a substrate cleaning device such as a batch type substrate cleaning device or a single-wafer type substrate cleaning device, or to a substrate treatment device such as a CMP device, and unused ultrapure water is returned to the inlet of the subsystem or the primary pure water system to be reused.
When the ultrapure water thus obtained is used as cleaning water for the semiconductor substrate, if the concentration of dissolved oxygen in the cleaning water is high, a natural oxide film is easily formed on the wafer surface by the cleaning water. This may block the precise control of the film thickness and the film quality of a gate oxide film, or increase the contact resistance of a contact hole, a via, a plug, or the like. In addition, a wiring metal such as tungsten or copper is exposed on the surface of the substrate that is to be cleaned after the polishing step in the formation process of the multilayer wiring. Since this wiring metal is easily corroded by the oxygen dissolved in the cleaning water, the wiring may be subjected to oxidation corrosion during the substrate cleaning which causes deterioration in the performance of the integrated circuit element that is to be created from the substrate.
As a countermeasure, a membrane degasifier (MD) may be installed in the subsystem for the purpose of removing gas components, in particular oxygen, dissolved in the ultrapure water. In general, the membrane degasifier is often installed between the ultraviolet oxidation device and the non-regenerative ion-exchange device (FIG. 3), or between the non-regenerative ion-exchange device and the membrane separation device (FIG. 4). The former case has an advantage in which impurities (ions) eluted from the membrane degasifier can be removed by the non-regenerative ion-exchange device. The latter case has an advantage in which the very small amount of gas components generated by the non-regenerative ion-exchange device can be removed by the membrane degasifier.
As another countermeasure, as described in Patent Literature 1, there has been proposed a method for reducing the dissolved oxygen in the water by adding inactive gas to the degassed ultrapure water. In Patent Literature 1, a method for replacing an atmosphere near the surface of the substrate to be cleaned with inactive gas is also employed. In addition, Patent Literature 2 discloses a method for dissolving hydrogen gas in the ultrapure water to prevent the oxidation of the substrate.
In the above-mentioned subsystem, since the water is irradiated with ultraviolet rays to decompose/remove the organic substances in the water by the ultraviolet oxidation device, water molecules are also oxidized by the ultraviolet irradiation to generate hydrogen peroxide that is an oxidizer. This means that the ultrapure water includes hydrogen peroxide.
As a method for removing the hydrogen peroxide in the water, a technology to remove hydrogen peroxide in the water by a platinum-group metal catalyst, such as palladium, as described in Patent Literature 2, is known. In particular, Patent Literature 3 proposes a method for highly efficiently decomposing and removing the hydrogen peroxide from the raw water at a high speed by bringing a catalyst prepared by supporting a platinum-group metal on a monolithic organic porous anion exchanger into contact with the raw water including the hydrogen peroxide. In the method for removing the hydrogen peroxide by the platinum-group metal catalyst such as palladium, as described in Patent Literature 2, it is known that the hydrogen peroxide and the dissolved oxygen can be simultaneously removed by dissolving the hydrogen before the water passes through the platinum-group metal catalyst.
Since the impurities have been removed, to the maximum degree, from the ultrapure water obtained by the above-mentioned subsystem, its resistivity reaches 18 MΩ·cm or more and has high insulation properties. When a substrate such as a semiconductor wafer is cleaned with the ultrapure water that has such high resistivity, charging of the substrate result from friction which causes fine particles attached to the substrate due to electrostatic attraction or which causes electrostatic destruction of elements due to discharging. As a countermeasure, as described in Patent Literature 4, a method for reducing the resistivity of water by adding highly pure carbon dioxide to the ultrapure water and using this carbon dioxide added water (i.e., carbonated water) as a cleaning solution is employed.
The properties of the carbonated water are different from those of the ultrapure water in that the resistivity is low and pH exhibits mild acidity due to a high carbonic acid concentration as described above. Accordingly, for example, when the carbonated water is used as cleaning water, it is known that a problem, in which copper exposed on the substrate surface is dissolved, occurs. With further miniaturization of the pattern dimensions of the integrated circuit element, the thickness of the wiring becomes smaller, creating a possibility that even slight wiring corrosion may reduce the performance of the integrated circuit element in the future.
Thus, as described in examples herein, the inventors have analyzed the test for cleaning the copper-exposed substrate with the carbonated water, and found that hydrogen peroxide contained in the carbonated water contributes to the corrosion progress of the copper. In the above-mentioned subsystem, the hydrogen peroxide is generated in the water by the ultraviolet oxidation device, and it has been found for the first time that this hydrogen peroxide is the cause of the corrosion.
In other words, the inventors have found that when the substrate is cleaned with the carbonated water prepared based on the ultrapure water irradiated with ultraviolet rays, the removal of the hydrogen peroxide in the carbonated water is effective for further preventing the corrosion of the copper or the copper compound exposed on the substrate surface.